1. Field of the Invention
This invention relates to a microwave sensor for detecting the presence of an object, and in particular a moving object.
2. Description of the Prior Art
Microwave sensors for detecting the presence of moving objects, and the direction of movement, are found in a variety of applications such as door openers, security systems, and toilet flushers. Such microwave sensor typically comprise a high frequency (or radio frequency) "RF" circuit comprising an oscillator for generating a high frequency oscillating signal, an antenna or antennas for transmitting and receiving the oscillating signal, and a mixer for modulating the oscillator signal with the received signal. In order to determine the direction of motion of an object, the oscillator signal is mixed with the receive signal to generate two output intermediate frequencies "IF" with 90.degree. phase difference, the combination of which determines the direction of movement of an object toward or away from the sensor. The velocity may be determined from either IF which both comprise the Doppler frequency component.
The RF circuit in microstrip outlay of a known microwave sensor is illustrated in FIG. 1. The RF circuit 102 of the known sensor comprises an oscillator 104, a transmission line 106, an antenna 108 for transmitting and receiving the microwave signals generated by the oscillator, and first and second mixer diodes 110, 112 respectively that are connected to the transmission line 106 at a spacing from each other corresponding to 1/8.sup.th of the wavelength of the oscillator signal. The 1/8.sup.th wavelength separation of the mixer diodes 110, 112 corresponds to a phase shift of 45.degree.. The oscillator power LO1 arriving at the first mixer diode 110 (D1) mixes with a receive signal (RF1) resulting from reflection of transmitter power off an object and received by the antenna 108. Similarly, oscillator power LO2 at the second mixer diode 112 (D2) mixes with a receive signal (RF2) . Because the signals LO1 and LO2 have a phase difference of 45.degree., and the receive signals RF1 and RF2 have a phase difference of -45.degree., overall the output intermediate frequency signals IF1 and IF2 have a phase difference of 90.degree.. One of the problems of this known arrangement, is that the power of the oscillator signals LO1 and LO2 may not be identical in view of losses along the transmission line, or due to varying characteristics of the diodes 110, 112. Similarly, the power of the receive signal RF2 may differ slightly from RF1 such that the outputs IF1 and IF2 differ. Power and diode variations also cause a relative phase shift of the signals IF1 and IF2. In order to process the signals IF1 and IF2 in view of determining the direction of motion or computing the velocity, the outputs IF1 and IF2 need to remain within certain tolerances. In prior constructions, it is often necessary to manually tune the sensor to compensate for the unequal power divisions and phase differences in the separate mixer diodes.
It would be desirable to improve the reliability of signal processing and reduce or avoid the need for tuning.